It is well known that the intensity, phase or spectrum of light can be controlled when an electric field is applied to an electro-optical crystal through which light propagates. (See, e.g., for example, A. Yariv, Optical electronics in modem communications, 5-th edition, Oxford University Press, 1997).
When such an electric field is provided with a modulation frequency, the output signal from the electro-optic crystal has an output frequency that comprises a set of equidistant spectral lines centered around the input frequency of the light. Therefore, modulation with a signal having a plurality of modulation frequencies will cause the output spectrum to contain a plurality of sets of frequencies.
FIG. 1 shows typical output spectrum of an output signal from an EOM (electro-optical modulator). The spectral line corresponding to the base, or central, frequency of the light signal has the highest intensity and the spectral lines 104, 106 on either side are lower, their intensities tapering with their distance from the central frequency 102, thereby giving an envelope 108 of intensities which rises and then falls, as a function of frequency. The rapid fall of the intensity for the spectral lines away from the input (central) frequency does not allow for the creation of a wide comb of frequencies having substantially same intensities, when just an EOM is used.
The prior art also discloses that electro-optical crystals, and in particular waveguides in electro-optical crystals, can be doped by elements which, upon pumping, produce a gain, thereby amplifying light radiation transmitted through the waveguide. U.S. Pat. No. 5,473,722, entitled “Rare-earth-doped Lithium Niobate Waveguide Structures” discloses amplifiers based on Ti:LiNbO3 waveguides doped with Erbium. This is one way to make light amplifiers and lasers.
FIG. 2 shows the comb for an Er-doped LiNbO3 mode-locked laser. As seen in this figure, the output spectrum of a mode-locked laser represents a comb of equally spaced frequencies. However, the spectral lines in the comb collectively form an envelope 202 that rises and falls with frequency. As also seen in this figure, the width of the comb envelope is less than 100 GHz, and therefore covers only a very small part of the 4 THz C-band range.
It is known, however, that operation of a mode-locked laser at a pumping power level lower than the lasing threshold prevents laser oscillation. Under sub-lasing pumping power conditions, a mode-locked laser operates as a modulator/amplifier. The exact gain distribution depends on such factors as the doping type, the doping level, and the pumping level. Rare-earth-doped waveguides are pumped by irradiation with wavelengths below the C-band to generate radiation at wavelengths in the C-band. Optical pumping of an erbium-doped waveguide can provide spontaneous radiation covering the entire C-band. The EO modulation enhances radiation from a portion of this band, and converts some of the energy in the highest gain wavelength into sidebands in the other frequencies. In this case the output spectrum represents a comb of frequencies spaced by the Free Spectral Range (FSR) determined by the EO modulator. The bandwidth of the output spectrum of a mode-locked laser operating at a sub-lasing threshold is therefore much wider than the output spectrum of a mode-locked laser operating above the lasing threshold.
FIG. 3 represents the output spectrum of an EO modulator operating below lasing threshold. The total bandwidth of output spectrum is about 800 GHz, and since the FSR (free spectral range) is 10 GHz, the total number of “teeth” in the comb is about 80. A comparison between FIGS. 2 and 3 shows that the output spectrum is wider when the gain medium in the modulator is operated below the lasing threshold (FIG. 3) than when the gain medium is pumped above lasing threshold in a mode-locked laser (FIG. 2).
Further spreading of the output spectrum is limited mostly by waveguide dispersion that leads to a velocity mismatch between the applied RF signal and the optical wave. One way to address the problem of waveguide dispersion is to introduce compensating dispersive elements inside the cavity, such as prisms or diffraction gratings of dielectric mirrors. Such an approach is described in L. R. Brothers et al. “Dispersion compensation for terahertz optical frequency comb generation”, Opt.Lett., 1997, v.22, no.13, pp.1015-1017.
It is also known in the prior art that one may create a variety of periodic structures in the crystal of an electro-optical modulator. One of the known techniques is a periodic poling (PP) in electro-optical crystals like in LiNbO3, LiTaO3, KTP or poled nonlinear polymer materials. This technique involves periodically inverting the crystal structure or domain on a micrometer scale. This is done in, e.g, PPLN (periodically poled LiNbO3) crystals produced by e.g., HCPhotonics, of Hsinchu, Taiwan.
A mode-locked laser may include an electro-optic modulator configured to modulate the refractive index of the laser cavity. For example, an integrated mode-locked laser comprising an electro-optically modulated Er-doped Ti:LiNbO3 waveguide is disclosed in H. Suche et al. “Integrated Optical Ti:Er:LiNbo3 Soliton Source”, IEEE J. of Quantum Electronics, 1997, v.33, no.10, pp.1642-1645. The output signal represents a series of short pulses in the time domain, or a comb of frequencies in frequency domain. The modulation frequency determines the FSR of the laser's output spectrum. The FSR is the frequency spacing between the “teeth” of the comb.
The prior art includes optical communication systems that incorporate light sources that output optical signals having evenly spaced frequencies. U.S. Pat. No. 4,989,201, entitled “Optical Communication System With a Stabilized “Comb” of Frequencies” is one such example.